Bounty hunters and private investigators, lured by a $2m (pounds 1.2m) reward, have joined FBI agents in the hunt for the elusive "John Doe 2", the unidentified suspect believed to have carried out the bombing in Oklahoma City with Timothy McVeigh, the first man arrested and charged.
Since Mr McVeigh's capture, within hours of the explosion that killed 168 people, the impression is that the hundreds of agents investigating the theory that he was part of a larger conspiracy have been beating their heads against a brick wall.
No more intensive search has been conducted in US police history. But, as a source close to the case said, the more time passes, the colder the trail gets.
Exasperated federal investigators have pondered whether John Doe 2 died in the explosion, whether he is a mistakenly identified 12-year-old boy, or whether he exists at all. They have received 35,000 leads following the release of a sketch of the suspect, but their toil has yielded little.
Mr McVeigh has been under 24-hour surveillance from a television camera in his cell since the day of his arrest. But he has provided no clues as to whether he was working with anyone else.
The second man arrested and charged in connection with the bombing, Mr McVeigh's old army friend Terry Nichols, is alleged to have implicated Mr McVeigh. According to an FBI affidavit, Mr Nichols said Mr McVeigh had told him "something big" was going to happen a week before the bombing.
But, as his lawyer has pointed out, Mr Nichols was at his home in Kansas when the explosion happened. The case against Mr Nichols appears to rest on the discovery of anti-government propaganda, chemicals and explosive components at his home.
In a damaging blow to the FBI investigation, Mr Nichols' brother, James, was released from jail without bail on Monday after four weeks under arrest. A judge threw out arguments by federal prosecutors that James Nichols, who was brought to court in leg-irons, was involved in the bombing. "There is not an iota of evidence that he is a danger to others," the judge said of Mr Nichols, described by neighbours as a hard-working farmer.
One other individual questioned by police has tightened the noose around Mr McVeigh but yielded nothing new. Michael Fortier told investigators that he and Mr McVeigh had travelled to Oklahoma City and gone inside the Alfred Murrah building one week before the explosion. The plan, according to Mr Fortier, had been to check out the layout.